In the recovery of latex solids, it is usually necessary or desirable to conduct the process so that the recovered solids will be substantially free of moisture and contaminants, such as emulsifying agents and water-soluble salts, and will have a particle size and particle size distribution suitable for subsequent compounding operations. In commercial-scale operations, of course, it is also desirable to conduct the process as economically as possible.
Many known techniques permit the efficient, economical recovery of high quality (i.e., substantially moisture and contaminant-free, suitable particle size) rubber solids from latices, but, as is well known, it is difficult to recover high quality solids from latices of polymers having heat distortion temperatures above 0.degree. C., without utilizing expensive and sometimes unduly time-consuming procedures. Thus, when known techniques are used to recover these resinous polymers from latices, it is frequently found that:
(1) The techniques which permit facile, adequate removal of water and contaminants lead to the formation of a high percentage of particles which are smaller than 60 mesh (designated as "fines-fraction") creating problems because (a) in the case of many polymers, an explosion hazard is created by a high concentration of fines in efficient drying equipment, (b) compounding problems are presented when a polymer which is to be mixed with one or more additives contains too high a percentage of fines, and (c) it is uneconomical to separate and discard the fines in order to avoid the problems which their presence creates.
(2) The techniques which insure a minimum formation of fines make it at least very difficult to accomplish adequate removal of water and contaminants-disadvantageous because of (a) processing problems presented when the polymer has too high a moisture content, (b) the deleterious effect of the contaminants on polymer properties, and (c) the expense of the equipment required to remove the difficulty-removable water and contaminants.
(3) The larger amounts of water usually present in the coagulum when it is fed to a drier necessitates the use of driers having a large load capacity-disadvantageous because of the greater expense of the larger load capacity driers.
An object of the invention is to provide a novel process for recovering latex solids.
Another object is to provide a process which permits the economical recovery of substantially moisture-free polymers having a desirable particle size and particle size distribution from latices of polymers having heat distortion temperatures above 0.degree. C.
Another objective is to provide an improved process wherein a fines-fraction of the polymeric particles is separated and recycled to said latex for incorporation in said solids particles.
U.S. Pat. No. 3,345,430 discloses a process for recovering graft copolymer latex solids having a reduced fines-fraction by coagulating said solids followed by synerizing and hardening said solids to reduce break-up during washing, granulation and drying, however, although the fines-fraction is reduced, further reduction has been found necessary to increase drying efficiency. The present invention provides a novel improved process wherein said fines-fraction is recycled and incorporated in said solids particles producing a more desirable particle size and particle-size distribution for drying and subsequent processing.